High frequency transformer



April 10, 1934. w. L. CARLSON HIGH FREQUENCY TRANSFORMER Original Filed. Jan. 16

Patented Apr. 10, 1934 UNITED STATES 1,954,470 HIGH FREQUENCY TRANSFORMER Wendell L. Carlson, Haddonfield, N. J., assignor to Radio Corporation of America, New York, N. Y., a corporation of Delaware Original application January 16, 1931, Serial No. 509,143. Divided and this application March 31, 1933, Serial No. 663,770. In Germany January 15 Claims.

My invention relates to radio receiving circuits and apparatus, and more particularly it relates to a circuit arrangement and apparatus therefor adapted to control the selectivity and fidelity of reproduction under differing conditions of recep- 'tion. In the broadcast band of frequencies, for example, a receiving apparatus may be required to receive signals or programs from a powerful local station requiring low gain with a broad tuning response for good fidelity, or from a distant station differing in frequency by only a few kilocycles from an undesired station, requiring a greater and even a maximum amount of both selectivity and gain.

With transmitting stations operating at frequencies differing from each other by only a few kilocycles it is diilicult, ordinarily, to obtain good fidelity simultaneously with good selectivity. To obtain a desired degree of selectivity in the one case and the desired degree of fidelity in the other, it is necessary to provide means for chang-. ing the receiving circuit constants for local as distinguished from distance reception.

This application is a division of my co-pending original application, Serial No. 509,143, filed J anuary 16, 1931.

Accordingly, itis an object of my invention to provide an improved and simplified circuit and apparatus therefor adapted for use in a radio receiving system whereby the same may be operated with either high gain and great selectivity or with reduced gain and increased fidelity of reproduction.

A further object of my invention is to provide an improved coupling or transformer unit for use in receiving systems between thermionic tubes which is simple in construction, easily adjusted and which is effective in a selectivityfidelity control circuit.

My present invention is in the nature of an improvement over the subject matter disclosed in my co-pending application, Serial No. 193,438 filed Mar. 23, 1927, now Patent 1,871,405. In the said application it is disclosed that fidelity and selectivity may be controlled by a switching arrangement which changes the relation between certain windings of a transformer in one or more points of a receiving system and, at the same time, changes the amount of resistance in the transformer circuit.

In accordance with my present invention, a

plurality of loosely coupled tuned circuits, preferably in the intermediate frequency amplifier of a superheterodyne receiver, are arranged to be changed from a condition of high selectivity and gain to a condition of reduced selectivity and gain by means for readily changing the amount of resistance in one or both of said circuits without disturbing the coupling relation between cir- 6 cuits.

Heretofore, it has been customary, in receiving systems including a transformer as a circuit coupling means, and the transformer having relatively close coupling between primary and secondary circuits, to damp the transformer by means of a resistance in either the primary or the secondary circuit. In such a case, as is well known, appreciable resistance is transferred to the opposite side of the transformer circuit, the amount depending upon the degree of coupling and voltage ratio of the transformer.

In my present circuit, however, by reason of the looseness of coupling between circuits, any resistance inserted into one circuit is substantially not transferred into the other circuit. Furthermore, since it is not feasible to obtain the desired reduction in gain by introducing an excessive amount of resistance into only one of the coupled circuits, I have found it desirable to provide means to introduce a limited amount of resistance into one circuit and simultaneously or subsequently to introduce a certain amount into the other circuit.

Other objects of my invention will become evident from the following detailed description, taken in conjunction with the accompanying drawing, in which Fig. 1 is a complete circuit diagram of a superheterodyne receiving system embodying my invention; Fig. 2 is a full-sized sectional view of a transformer or coupling unit used in the circuit of Fig. 1, and Fig; 3 is a full-sized sectional view, similar to Fig. 2, of another coupling unit in the circuit of Fig. 1. 1

Referring to Fig. 1, an antenna 1 is coupled to loosely coupled variable tuned circuits 3 and 5 which impress selected radio frequency signals upon a screen grid thermionic amplifier tube 7. The output circuit of the tube '7 is coupled to the first detector 9, of the screen grid type, by suitable means such as a radio frequency choke coil 11, a condenser 13, and a variable tuned circuit 15 providing the input circuit therefor. An oscillation generator, comprising a tube-17, a feedback coil 19 and a variable tuned circuit 21, is coupled to the input circuit of said detector.

The detector output circuit is connected with a coupling unit including a coil 23, tuned by means of a semi-variable condenser 25 to the desired intermediate frequency. A coil 27, tuned by a condenser 29 in like manner, is loosely coupled to the coil 23 with a coupling no greater than optimum. A disc or annular ring 24,-of thin current-conducting material, preferably of low resistance material such as copper, is disposed between the coils 23 and 27, having a common axis therewith, and eiiects very loose coupling with relatively closer spacing of coils than would otherwise be possible. Such an arrangement may be used to advantage in radio frequency transformers as well;

An intermediate frequency amplifier screengrid tube 31 is coupled to a similar succeeding intermediate amplifier tube 33 by a plurality of loosely coupled resonant circuitsincluding coupling coils 35 and 37. Similarly, the tube 33 is connected with the second detector 39 by a pair of loosely coupled circuits including coupling coils 41 and 43. The selectivity of each of the latter pairs of coupled circuits is less, and the coupling is greater, than that of the coupled circuits in output of the first detector tube 9. No shielding device is used between the coils of circuits 35 and 37 and the coils of circuits 41 and 43 while the physical spacing between coils is less than in the case of the coils 23 and 27. The coupling unit used between said pairs of coupled circuits including coils 35, 37 and 41, 43 is illustrated in structural detail in Fig. 3, described later, and in connection with Fig. 2. k

A double-throw, double-pole switch 69 is provided for throwing resistance into and out of each of a pair of the loosely coupled circuits such as 23, 25 and 27, 29. The circuit arrangement is such that in the switch position shown, resistor 74 of the order of 40,000 ohms is shunted across circuit 23, 25, while resistor 76 of the order of 500 ohms is caused to be in series in the tuned circuit 27, 29. In. the lower position of the switch, resistor '74 is open-cncuited and resistor 76 is short-circuited.

Referring particularly to Fig. 2, the physical relation of the coils 23 and 27 and the disc or partial shield 24 is shown, all being mounted coaxially upon a supporting core 26 of insulation material, the copper eddy current shield disc or short cir= cuited ring being disposed between the coils, preferably centrally positioned. However, the position of the disc along the support 28 is not critical with regard to coupling between coils 23 and 27. It is desirable that disc 24 be properly spaced from either coil to. avoid interfering with the circuit constants. It will'be noted that the coils are fixed upon the support whereby the coupling relation is fixed. This is desirable in quantity production. However, the shield member provides ready means for adjusting the coupling, it being necessary only to bend down the edge of the member or disc at one or more points, shown in dotted lines at 24', to increase the coupling between coils. As above stated, this provides a simple and inexpensive means for adjusting coupling in factory production while permitting the coils to be permanently mounted.

The fittings or pins 28, Fig. 2, used to secure the core to the brackets 30 are preferably of low resistance current-conducting non-magnetic material such as copper or brass, in order to avoid an undue increase of losses and to permit sharp tuning. Similarly the shielding container 32 is of copper. However, in the succeeding intermediate coupling units, shown structurally in Fig. 8, the shielding containers indicated at 82 in dotted lines, and certain of the fittings such as pins 83 and 89, are preferably of a higher loss material such as iron to lessen the sharpness-of tuning. Iron, because of its magnetic properties in addition to its higher resistance nature, is particularly effective in increasing losses to accomplish the foregoing effect. However, in one combination,-

e. g. in transformer 35--37, the pin 38, Fig. 3,

while pin 89, used with secondary 37, may be of lower resistance material such as brass. In the foregoing coupling units, the presence of the above metallic elements, including the shielding container, tends to effect a small decrease in sharpness of tuning due to the fact that the resistance to inductance ratio of the coils is increased.

Notwithstanding the fact that I have illus- 85 trated my improved gain-selectivity control in its application to a pair of coupled circuits in the signal channel of a radio receiver representing any signal transmimion system, it is obvious that three or more coupled circuits may be em- 9 ployed, using means to. introduce resistance into all three or into two of the three, as for example the first and third. It should also be obvious that the circuits so controlled may be those in other parts of the system as for example in any 5 of the other intermediate frequency amplifier stages or in any of the radio frequency portions, or in both. For reasons concerning the control of volume it is desirable in reducing amplification for local reception to effect this reduction 0 at a point about midway of the cascade amplifier system. In the circuit shown the first intermediate frequency amplifier stage is preferred for practical reasons.

From the foregoing description it will be seen that in a radio receiving apparatus having a signal transmission channel there is interposed in said channel a coupling unit or transformer having coupling coils arranged to provide electro magnetic coupling between them, such coils being included in successive adjacent circuits and being arranged preferably coaxially.

Furthermore, it will be seen that the degree of coupling between said coils and hence, between the circuits in which they are included, is made relatively less, whereby a high degree of selectivity is obtained for signals transmitted through said coupling device. Furthermore, each of said coils, or circuits in which they are included, are tuned to a desired frequency which, in the present example, is the intermediate frequency of the superheterodyne receiver.

The shielding device or copper ring or disc interposed betweenthe coils of the coupled circuits provides a higher degree of selectivity than is possible with a mere physical separation of the coupled coils. In other words, the shielding device serves to electrically isolate or separate the coils to a degree depending upon the size and shape of the device.

It will be appreciated that an arrangement or. this character is particularly well adapted for production manufacturing methods since the shielding device may be mutilated or deformed, for example by being bent at its outer edge, to adjust the degree of coupling without moving or adjusting the positions of the coils. The coils may thus be permanently mounted in position without further adjustment or change. 7

The coupling device itself is of simple construction including a central mandrel or core of insulating material which forms a support and centering means for the coils and for the shielding device. Furthermore, the mounting means supports are utilized to vary the response characteristics of the coupled circuits. Thus the brads or pins pressed through the supports and into the central mandrel maybe of diii'ering materials, as hereinbefore described, thereby serving a dual purpose.

Although I have shown and described certain specific embodiments of my invention, I am fully aware that many modifications thereof are possible. My invention, therefore, is not to be restricted except insofar as is necessitated by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A coupling unit for a radio receiving system including a pair of coupling coils, a support on which said coils are mounted in spaced relation to each other, a short circuited ring member of low resistance material mounted between said coils, fastening means for said support disposed within the field of one of said coils, said last named means being of low resistance material for minimizing its effect upon the high frequency resistance of said coil.

2. A coupling unit for high frequency circuits including a pair of coupling coils, an elongated support of insulating material on which said coils are mounted in spaced coaxial relation to each other, a low resistance metallic annular member mounted upon said support between said coils and axially spaced therefrom, said member comprising a portion of substantial area in a plane perpendicular to the axis of said support and a cylindrical flange for securing said member to said support.

3. Ahigh frequency transformer comprising a pair of fixed coupled coils, an insulating support on which said coils are mounted in spaced coaxial relation to each other, and a metallic ring partial shield member mounted on said support between said coils for decreasing the coupling between coils, and a low resistance casing around said coils providing an external shield for said transformer.

4. A high frequency transformer including a pair of cylindrical coils, a support on which said coils are mounted in spaced coaxial relation to each other, a short circuited metallic ring shield member mounted on said support between said coils for decreasing the coupling between coils, said ring being of thin conducting material and being adapted to be deformed along its edges to adjust the coupling between said coils.

5. A high frequency transformer including a pair of coils mounted in spaced coaxial relation to each other, a support for said coils, a pair of fastening means secured to said support, one of said fastening means being of higher resistance material and being disposed in the field of one of said coils, and the other of said fastening means being of lower resistance material and disposed in the field of the other of said coils.

6. A high frequency transformer including a pair of coils mounted in spaced relation to each other, a support for said coils, a pair of damping elements secured to said support, one of said elements being of relatively higher resistance material and being disposed in the magnetic field of one of said coils, and the other of said elements being of lower resistance material and disposed in the field of the other of said coils.

7. A high frequency transformer including a pair of coils mounted in spaced coaxial relation to each other, a support for said coils, an insulated metallic ring mounted on said support between said coils, a pair of fastening means secured to said support, each of said fastening means being of low resistance material and disposed in the field of each of said coils respectively, and a low resistance metallic shield casing for said transformer.

8. A high frequency transformer including a pair of coils mounted in spaced coaxial relation to each other, a support for said coils, an insulated metallic ring mounted on said support between said coils, a pair of damping means secured to said support, each of said means being disposed in the field of each of said coils respectively, and a low resistance metallic shield casing for said transformer.

9. In a high frequency transformer, a primary coil and a secondary coil coupled magnetically therewith, said coils being arranged in spaced coaxial relation to' each other, a shield member of low resistance material mounted adjacent said coils and having edges adapted to be deformed to vary the coupling between said coils.

10. In a transformer for a high frequency selective signal system, a primary coil and a secondary coil loosely coupled therewith, an element of high resistance material operatively associated with said primary coil, and an element of relatively lower resistance material operatively associated with said secondary'coil for controlling the selectively of said system.

11. In a coupling unit for a high frequency selective circuit, a plurality of spaced coils for coupling circuits with a coupling no greater than optimum, and eddy current means interposing a partial shielding effect between said coils for reducing the coupling therebetween.

12. In a high frequency transformer for coupling circuits in a selective system, a pair of magnetically coupled coils, said coils being relatively closely spaced in substantially coaxial relation, and means comprising an eddy current shield so positioned with respect to the fields of said coils as to interpose a partial shielding effect between said coils and to provide looser coupling between said circuits than would be possible in a given space, with said coaxial relation, in the absence of said shielding means, whereby a high degree of selectivity is obtained.

13. In a high frequency transformer for coupling circuits in a selective system, a pair of magnetically coupled coils, said coils being relatively so positioned as to provide a substantial degree of coupling between said circuits, and means comprising an outer shield casing of low resistance material and a shield member between said coils, so positioned with respect to said coils as to loosen the coupling between said circuits to a degree substantially no greater than optimum.

14. In a high frequency transformer for coupling circuits in a selective system, a pair of magnetically coupled coils, said coils being relatively so positioned coaxially in fixed relation to provide a degree of coupling between circuits that is too great for desirable high degree of selectivity, and a low resistance eddy current ring so positioned coaxially with respect to said coils as to interpose a partial shielding effect between said coils, thereby to loosen the coupling between said circuits to a degree substantially that of optimum coupling.

15. A high frequency coil assembly comprising in combination, a multiple layer coil, said coil having a non-magnetic core, a non-magnetic conductor extending into the core of said coil whereby eddy currents are generatedin said conductor, a shield for shielding said coil, said conductor and said shield co-operating to maintain a high ratio of inductance to resistance of the coil.

WENDELL L. CARLSON. 

